1. Field of the Invention
The present invention refers to a new process for the manufacturing of flame retardant (FR) viscose, this understood as being regenerated cellulose.
2. Description of the Related Art
The first flame retardant viscose fibre (regenerated cellulose) of which knowledge is available was produced in Finland by Kemira in 1991 and was commercialized under the name “Visil®”, this becoming Avilón FR in recent years.
In recent years, other producers of flame retardant viscose have emerged, using the same technique as that utilized by Kemira.
This technique consists of producing the flame retardant viscose by adding silica to the viscose and subsequently fixing the silica to the fibre. By adding silica to the viscose, the final flame retardant fibre will incorporate regenerated cellulose and a silica polymer providing the fibre with flame retardant properties.
This silica polymer is fixed to the fibre by using aluminum sulphate in the subsequent bleachings of the latter, this allowing this type of fibre to maintain its flame retardant properties even after the aggressive action of alkaline detergents. The use of aluminum sulphate allows the flame retardant properties to be maintained over time.
The silica is proportioned over the viscose en masse, following a preliminary process of preparation. The raw material used is superneutral sodium silicate or soluble glass with a weighted gradation of 3.3/l. The manner in which this raw material is used is 3.3SiO2×Na2O. This product is a viscous, transparent and colourless liquid that is alkaline in reaction and soluble in water at all proportions.
In order to avoid gelling or solidification, the aforementioned product is diluted in electrolytic soda, which is equally diluted.
Following the preparation of the silica, it is proportioned over the viscose en masse. The virgin or pure viscose has the following approximate composition:                Cellulose (C6H10O5) at approximately 9.36% by weight.        Soda (NaOH) at approximately 5.5% by weight.        Carbon sulphide (CS2) at approximately 3.5% by weight.        Water at approximately 81% by weight.        
The proportioning of the additive must be such that the viscose mixture has a content of silica with respect to the cellulose in the viscose of approximately 50%.
The viscose mixture is then regenerated, for which the said viscose is extruded via very small diameter spinnerettes in a spinning bath, this promoting the coagulation of the viscose and the additive.
The conventional spinning bath over which the viscose is coagulated is known as a Muller bath and has the following composition:
Density1305.0gr/lH2SO4120.0gr/lSO4Zn7.5gr/lSO4Na2323.7gr/lH2O853.8gr/lTemp.48.0°C.
The regenerated fibre is subjected to the action of a bleaching agent, specifically sodium hypochlorite (NmOCl) and subsequently washed with an antichlorine (H2SO4).
Although it provides satisfactory results, this process poses the problem of its contaminating effect, in particular due to the use of zinc in the coagulation bath and of sodium hypochlorite and sulphuric acid in the bleaching phase, since this implies the use of highly contaminating heavy metals and organochlorated compounds.